The present invention relates to a tuning circuit for broadcast receivers, particularly of the type employing a digital input keyboard for reading in of the desired tuning frequency and circuitry for internally storing the information for timing to preselected stations.
U.S. Application Ser. No. 708,754, filed by Otto Klank, Dieter Rottmann and Stephan Wuttke on July 26th, 1976, discloses a tuning circuit for high frequency, broadcast, receivers which includes a voltage controlled superheterodyne oscillator whose frequency can be varied by means of a tuning voltage to tune the receiver to a desired station. The tuning circuit includes a comparison circuit which compares representations of the digits of two numbers to produce the tuning voltage. One of the numbers represents the counting state of a counter which periodically counts the oscillations of the superheterodyne oscillator bearing a fixed relation to the intermediate frequency. This number thus constitutes the value of the received frequency. The other number is fed to the comparison circuit via an input keyboard provided with a series-connected coder. By punching appropriate keys of the input keyboard, that number which represents the station to which the set is to be tuned is introduced into the tuning circuit. From the deviation between the two numbers, as determined in the comparison circuit, the tuning voltage is derived to tune the receiver to the station represented by the other number.
In the tuning circuit disclosed in the above-cited pending application, a single comparator is provided which is capable of comparing the representation of only one digit, or position, of each of the two numbers at a time. This comparator receives the representations of each position, i.e. each order value or significance level, of the two numbers to be compared in succession, in multiplex operation, beginning with the most significant digit of each number, assuming that the two numbers have the same highest order of significance. Multiplex operation offers the advantage, inter alia, that only a few connecting lines are required for the comparison circuit. Moreover, only one memory is required, which in the above-cited pending application is called a circulating memory and is provided with a feedback path from its last memory cell to its first memory cell. The second number fed in via the input keyboard continuously circulates, in coded form, through this memory. From the last cell of the dynamic memory, the digits of the fed-in number reach the comparator.
The above-cited pending application also discloses the possibility of providing a station memory which can be selectively switched into the above-mentioned circulating memory feedback path, the station memory being capable of storing representations of a station frequency and band.
The circuit disclosed in the above-cited pending application incorporates known techniques for tuning receivers by means of a digital input keyboard, with digital frequency indication, of the type disclosed in the periodical "Funkschau", 1974, at pages 62-63 and 93-95.